Apparent and inherent optical properties in the ocean

Transcription

1 Apparent and inherent optical properties in the ocean Tomorrow: "Open questions in radiation transfer with a link to climate change 9:30 Gathering and Coffee 9:50 Opening Ilan Koren, Environmental Sciences and Energy Research, WIS 10:00 Warren Wiscombe, NASA - Goddard Space Flight Center, USA What is the minimum ante for playing the radiation game in climate models? 11:00 Alexander Marshak, NASA - Goddard Space Flight Center, USA From ICESat and MODIS to ARM; how radiative transfer helps to interpret satellite and ground-based measurements 12:00 Lunch break 13:30 Emmanuel Boss, School of Marine Sciences, University of Maine, USA, The backscattering enigma: what contributes t to oceanic backscattering? 14:30 Eli Tziperman, Department of Earth and Planetary Sciences, Harvard University, USA, Dinosaur forecast: cloudy (warming of the high latitudes by a convective cloud feedback) 15:30 Alex Kostinski, Department of Physics, Michigan Technological University, USA, Extinction vs. Spatial distribution of Scatterers

2 Review: In a plane-parallel medium without internal sources for a given wavlength: cosθ dl(θ,φ)/dz = - c(z) L(z,θ.φ) + 4π β(z,θ,φ;θ,φ ) L(θ,φ ) dω Which we can rewrite as: μdl(θ,φ)/dz = - c(z) L(z,θ,φ) + b(z) 4π β (z,θ,φ;θ,φ ) L(θ,φ ) dω μdl(θ,φ)/dτ = - L(τ,θ,φ) + ω 0 (τ) 4π β (τ,θ,φ;θ,φ ) L(θ,φ ) dω media with the same ω 0 and boundary conditions will have the same radiance distribution at similar optical depth horizons.

3 Some things to note about transmission, optical depth and tt ti attenuation: ( ) Δ 2 ) ( d r ( ) Δ = = = ) ( r c r r c cdr r r r τ Δ = = ) ( e e r r t N r c τ Demo (Petty): + = 1 1 1, 1 N i i N τ i τ Demo (Petty): effect of dilution on τ = = + = = 1 1 1, 1 1 N i i i e t t N i τ dilution on τ = + = = 1 1 1, 1 i e t t i i i N

4 Apparent Optical Properties usually: ratios of radiometric properties p (L, E, etc ) or normalized depth derivatives of radiometric properties. Why AOPs? IOPs are not always easy to measure. Radiometric properties depends strongly on sky conditions. Relatively l insensitive OPs Similar slopes Mobley, 2004: Hydrolight runs with 1 mgchl./m 3, with sun at 0, 30, 60 in clear skies and overcast skies.

5 Apparent Optical Properties: ratios of radiometric quantities Diffuse Attenuation, K (m -1 ) E d ΔE d = -E d K d Δz Dept th (m) z z 0 de d /E d = 0 K d (z) dz E d (z) = E d (0) e- Kd(z) dz Only when K d is not depth dependent E d (z) = E d (0) e -Kd z

6 Apparent Optical Properties Diffuse attenuation, K (m -1 ): Note that c and K are very different because K depends Upon the properties of the solar source θ z Δz K d1 K d2 Uniform c z+δz K d describes the loss of E d from z to z+δz, but the pathlength traveled is r = Δz/cos θ K d2 > K d1 K d < c

7 Apparent Optical Properties: diffuse attenuation coefficients How do different attenuation compare: Asymptotic regime, where all k s are equal Mobley, 2004, Hydrolight with Zaneveld et al., mgChl./m 3

8 Changes in spectral light penetration with depth for different water bodies. What causes the difference?

9 Apparent Optical Properties Average cosines describe the angular distribution of the light field: μ d = E d /E od μ u = E u /E ou μ = E/E o

10 Apparent Optical Properties Average cosines describe the angular distribution of the light field: μ d = E d /E od μ u = E u /E ou Solar beam μ = E/E o = (E d -E u )/(E od +E ou ) μ d = cos θ s μ u = 0 θ s μ = cos θ s

11 Apparent Optical Properties Average cosines describe the angular distribution of the light field μ d = E d /E od μ u = E u /E ou μ = E/E o = (E d -E u )/(E od +E ou ) μ d = ½ (θ=60 o ) μ u = - ½ (θ=120 o ) μ = 0 Isotropic light field

12 Average cosines μ d = E d /E od μ u = E u /E ou μ = E/E o 0.25m 1.25m b=15m -1,θ=32 Radiance distribution with depth

13 Average cosines The more scattering the faster is asymptotic attained The more absorbing the more collimated μ d = E d /E od μ u = E u /E ou μ = E/E o Mobley 2004 Hydrolight with b=a (b/c=0 5) & Mobley, 2004, Hydrolight with b=a (b/c=0.5) & b=4a (b/c=0.8).

14 Apparent Optical Properties Reflectance ratios (upward to downward): E u (0 - )/E d (0 - ) E u (0 - )/E d (0 + ) L w (0 + )/E d (0 + ) L u (0 - )/E d (0 + )

15 Apparent Optical Properties Reflectance ratios (upward to downward) Sample spectra

16 Not all reflectances are equally good: Mobley, 2004, Hydrolight with chl.=0.1,1,10 and sun angle, 0, 30 & 60 Rrs is less dependent on sun angle compared to R, a better AOP.

17 Absorbers and scatterers in ocean/lakes Water Phytoplankton Non-algal organic particles (CPOM) Dissolved organic matter (CDOM) Inorganic mineral particles (CPIM) Note: most available information is in the visible iibl ( ( nm). Why?

18 Absorption by (liquid and frozen) water: Assumes no internal sources. Features: Extremly complex Variety of vibrational modes Combination possible &

19 I It s not really the molecules l Density inhomogeneities Water clusters Water clusters with salt Unlike Rayleigh ~ λ (Morel, 1974)

20 Phytoplankton Bidigare et al 1991

21 Phytoplankton vs Chlorophyll Sosik & Mitchell chlorophyll chloroplast cell Packaging: a/[chl] is function of size and [chl] Duysens (1956)

22 Phytoplankton Species-specific pigment composition a( )/a a(676) Cell size Wavelength (nm) (Morel and Bricaud 1981)

23 Model for attenuation and scattering: ( λ) ( λ ) b b b b, p = b, p 0 λ λ 0 γ IOPs of phytoplankton (Bricaud et al., 1983, L&O):

24 Optical properties and HABs: Phyto. blooms Prorocentrum micans (27μm) Photo M. Keller Roesler and Etheridge, 2002 Aureococcus anafragefferens (3μm) Photo S. Etheridge

25 Non-algal organic particles Heterotrophic Ciliates, Flagellates, and bacteria Detritus Iturriaga and Siegel 1989 Ahn and Morel a CPOM (l) = a CPOM (400)*exp(-S CPOM (λ-400)) S CPOM = to 0.011

26 Scattering: Average spectral shape: Phytoplankton: Non algal particles: From Babin et al., 2003

27 CDOM: chromophoric dissolved organic matter a CDOM (l) = a CDOM (400)*exp(-S CDOM (λ-400)) S CdOM = to Kirk 1983 Carder et al. 1989

28 CDOM/Colloids scattering. 135 o 90 o 45 o Mostly negligible g in the ocean 180 o 0 o 135 o 90 o 45 o β(θ) b(λ) Example for water ~ λ -4 VSF θ Wavelength (nm) Similar results for viruses (Balch et al. 2000)

29 β(θ) response to particle size distribution First let s talk about particle size distributions r -3 r -5 Stramski and Kiefer 1989

30 β(θ) and response to particle size distribution / V p β(θ) /

31 β(θ) response to index of refraction β(θ) / V p

32 Theoretical dependence of the ~ backscattering ratio ( b ) bp bbp bp on index of refraction (n) and size (ξ): Twardowski et al., 2001

33 Backscattering ratio (55,000 observations from NJ shelf): consistent with theoretical prediction. Varies from: phytoplankt on inorganic particles.

34 Backscattering ratio relates to the relative concentration of phytoplankton and particulate material: Beware of photoaclimation!

35 Inelastic scattering: ( λ) = F ( λ) + F ( λ) F ( λ) F + w dissolved pigments Over all fluorescence is much weaker than absorption: O(1%) of absorbed photon fluoresce. Important in the clearest waters and water rich in phytoplankton (New paper, Behrenfeld et al., 2009 in Biogeosciences). F w - Raman Scattering (no consensus, Mobley, 1994). About 0.1b w. F d -Less well known. Variable shape and quantum yield. F p -known but depend on pigment composition F p known, but depend on pigment composition (chlorophylls, phycobillins). Variable quantum yield depend on physiology. F chl line height is a product of MODIS.

36 Conclusions The dominant components in the ocean have characteristic absorption and scattering coefficients We use the optical coefficients i as proxies for the component so as to determine concentration and composition (another lecture) While we measure bulk optical properties, there are methodological and modeling approaches to separate the constituents (another lecture)